1. Field of the invention
The present invention relates to novel benzothiophene/indole-substituted maleimide derivatives, a photochromic material thereof and an optical recording material using said material.
2. Description of the Prior Art
Photochromic materials that develop color or extinguish color upon irradiation with light have heretofore been extensively used for a variety of applications such as optical filters like sunglasses, and as masking materials and display materials.
A variety of organic compounds have been known that serve as photochromic materials, such as spiropyran-type compounds (Japanese Patent Publication No. 28892/1970). According to Japanese Laid-Open Patent Publication No. 24245/1988 filed by the present inventors, furthermore, diheterocyclic-substituted ethene derivatives such as a 1,2-di(2,3,5-trimethylthienyl) maleic anhydride and the like have been proposed as photochromic materials.
The photochromic material develops color and extinguishes color due to the isomerization of a molecular structure accompanying the irradiation with light, and must satisfy such requirements as a high quantum yield for developing or extinguishing color, development of a vivid color, a large contrast between color developing and color extinction, and relatively stable thermal state for developing color or extinguishing color.
The present inventors have previously synthesized maleimide derivatives having a benzothiophene group and an indolyl group at the first position and the second position of a maleimide, i.e., synthesized the maleimide derivatives of the following formula ##STR3## wherein R is a cyanomethyl group, R.sup.1 and R.sup.2 are methyl groups, respectively, and a hydrogen atom or a methoxy group is present at an ortho position with respect to a nitrogen atom in a ring B, and have discovered that the above derivatives are useful as photochromic materials.
These compounds are made compatible in a polymer (containing benzene), formed into a film (ring-opened product) to obtain a recording material, and are rendered to be a ring-closed product with visible light of 200 nm to 490 nm and are colored to write data therein. With this compound, however the isomerization factor from a ring-opened product into a ring-closed product (cyclohexadiene product) is still low, the contrast is low, and isomerized molecules are poorly maintained. Therefore, this compound is still not practical for use as an optical disk or an optical recording medium.